The measurement of very small magnetic fields is very important for biomedical research, e.g., for MCG (magneto-Cardiogram, fields smaller than 1 nanoTesla) and for MEG (Magneto-Encephalogram, fields smaller than 10 picoTesla). Currently these magnetic fields are measured with SQUID devices—Superconducting Quantum Interference Devices (SQUID) are very sensitive magnetometers used to measure extremely small magnetic fields (up to femtoTesla). These SQUID devices are only working at cryogenic temperatures and are extremely expensive, prohibiting their use in current medical practice.
The working principle Faraday rotation is shown schematically in FIG. 1. Faraday rotation is the rotation of the polarization plane of linearly polarized light due to magnetic field induced birefringence by a magnetic field parallel with the direction of light propagation. Faraday rotation has been known for more than hundred years and is commonly used in optical isolators. The rotation of the polarization plane (θ) is given by the product of the magnitude of the magnetic field (B), the sample length (L) and the Verdet constant (V): θ=VBL. The Verdet constant, which quantifies the Faraday effect, is for all materials different from zero and strongly wavelength dependent.
The principles of operation of a Sagnac interferometer are described in “The optical fibre Sagnac interferometer: an overview of its principles and applications”, B. Colshaw, Inst. of Physics Publishing, Meas. Sci. Technol. Vol. 17, (2006) R1-R16.
The article by Jing Xia, et al. entitled “Modified Sagnac interferometer for high-sensitivity magneto-optic measurements at cryogenic temperatures”, Appl. Phys. Letters, vol. 89, 06258 (2006) describes a zero-area Sagnac interferometer loop for measuring the magneto-optic Kerr effect at cryogenic temperatures. A beam of light from a broadband source is routed by a fiber to a polarizer and from there to a half-wave plate and an electro-optic modulator and from there to a fiber. The light emitted from the end of the fiber is reflected off the sample which is maintained at cryogenic temperatures.
The most common way of constructing a fiber Sagnac interferometer is by using the lock-in detection based modulator-demodulator scheme. The detection bandwidth of the lock-in scheme is limited by the modulation frequency, and this scheme requires costly optical and electronic components such as fiber-pigtailed optical phase modulator and a lock-in amplifier.
MEMS-based magnetometers are known that are used to measure relatively strong magnetic fields above microTesla level, and ultra-sensitive spectroscopy-based devices and SQUIDS. The MEMS magnetometers are extremely compact and inexpensive, but their sensitivity is limited. They are not suitable for biomedical applications. In addition, MEMS magnetometers are sensitive to effects of electromagnetic interference.
Atomic magnetometers (such as optically pumped magnetometers) have sensitivity of up to a picoTesla, but they are very fragile and expensive. They utilize several narrow-line, stabilized single-frequency laser sources for interrogation of atomic transitions in gas cells. SQUIDs that presently have the highest sensitivity of all magnetometer types are very expensive and impractical as they have to be operated at cryogenic temperatures.